In recent years, “stereo cameras” have been widely used in various fields, such as robot industries, or the like, due to the fact that the stereo cameras can measure three-dimensional space, and can help to research or identify the object. The stereo camera is structured of two cameras (lenses), each of which is placed at two separated positions to photograph the same object at the same time, whereby the stereo camera can acquire information regarding the depth direction, that is, an image can be acquired from which it is possible to stereoscopically acquire a space, as when a person directly views the object.
Since both cameras are positioned with each other in parallel against the horizontal line, said stereo camera can acquire two advantages.
A first advantage is that when a distance in the depth direction is to be measured, the fundamental principle of triangulation is used, if said distance is represented by Z, said distance Z can be easily acquired by Formula (10), shown below. However, in order to satisfy the relationship shown by Formula (10), both cameras are necessary to be placed parallel to each other, and if either camera is not placed in parallel, very complicated calculations are necessary to be conducted.Z=b/d×f  (10)
In Formula (10), “b” represents the distance between both cameras, “d” represents the positional disagreement on an imaging surface acquired by each camera with respect to the same object, and “f” represents the focal distance.
A second advantage is that within the two separate images photographed by said two cameras, any point on the image photographed by one camera can be easily identified as a corresponding point on the image photographed by the other camera. That is, on the stereo camera, an observing point, which is on the image photographed by one camera, is necessary to be searched on the image photographed by the other camera, as an observing point on the image photographed by the other camera, corresponding to that of the one camera, and a line for said searching is termed as an “Epipolar line”. If both cameras are arranged in parallel with each other, said Epipolar lines on one image and the other image are made consistent with each other in a parallel relationship, so that a point, corresponding to the observing point on the image photographed by one camera, is easily searched on the image photographed by the other camera.
However, from a physical point of view, it is very difficult to arrange two cameras to be truly parallel, whereby in recent years, the images photographed by two cameras are processed so that paralleling is established (for example, see Patent Documents 1 and 2).    Patent Document 1: JPA H7-294,215    Patent Document 2: JPA H11-203,476